In the following description and claims, the term "target plane" of a heliostat field means a plane in which the heliostat field forms an image of the sun, which is either the focal plane of the heliostat field or a plane conjugate therewith such as, for example, the focal plane of an optical system including the heliostat field and one or more additional reflectors which redirect the solar radiation concentrated by the heliostat field in direction of a solar receiver. In most cases, the target plane coincides with the radiation inlet of a solar receiver chamber. The term "alignment" used in relation to the heliostat field signifies the orientation of heliostat mirrors and/or individual facets thereof, if any, so as to focus reflected and concentrated light into the target plane and, in particular, onto the radiation inlet of the solar receiver chamber.
A conventional solar energy plant of the kind specified, such as disclosed, for example, by F. Ramos et al. in "Optimization of a Central Receiver Solar Electric Power Plant by the Aspoc Program", Proc. 4-th. intern symp. on research, development and applications of solar thermal technology, New-York, pp. 61-70, 1990, comprises a solar energy receiver placed within a chamber and a heliostat field in the form of a large Fresnel reflector having a target plane and consisting of a plurality of concentrating heliostat mirrors of a generally concave shape. Each heliostat mirror reflects the incoming solar radiation towards the target plane and there is thus formed thereby in the target plane a plurality of light spots, which together constitute a zone of concentrated solar radiation delivered by the heliostat field to the inlet of the receiver chamber. Each heliostat mirror is either a single body or is made of segments or facets having a specific mutual orientation which may be different for different heliostats of the field. During operation the angular position of each heliostat mirror at a given time is controlled by computer means whose program adjusts automatically to the position of the sun.
The efficiency of the solar energy plant, i.e. the conversion rate of solar energy into utilizable heat or electric power, depends on the amount of solar radiation collected by the heliostat field as well as on the solar energy concentration achieved at the inlet of the solar energy receiver chamber. The amount and concentration of radiation delivered to said inlet depend to a large extent on the accuracy of alignment of the heliostat field with the inlet of the solar receiver chamber. In other words, the higher the alignment accuracy of the heliostat mirrors and of the adjustment of their facets, the better the overlap of the individual light spots produced by different heliostat mirrors and by different facets thereof in the target plane and, consequently, the higher the amount and concentration of light delivered at the inlet of the solar receiver.
It is, therefore, clear that during operation of a solar plant of the kind specified, the orientation of the heliostat mirrors and the disposition of their facets have to be continuously controlled and adjusted in response to changing conditions, in order to maximize the amount and light delivered to the receiver chamber inlet.
It has been suggested to control the orientation of each heliostat mirror on the basis of detection of the direction of light reflected by the mirror, by means of a detector attached thereto. However, such a control system requires a high number of detectors each of which has to be properly aligned with respect to both the target plane and mirror. Furthermore, such a system cannot provide for correction of errors caused by misalignments of the facets, or dislocation of the target plane or of any of the detectors.
Conventionally, the initial alignment of the heliostat field and of individual heliostat mirrors thereof is based on measurements of the intensity of the light spots in the target plane. However, this procedure is inefficient because the overlapping light spots simultaneously produced by the heliostat mirrors and which together constitute a zone of concentrated solar radiation, cannot be identified individually and consequently the heliostat mirrors have to be aligned manually one after the other by qualified personnel. Obviously this procedure is very slow and has the further drawback that during the alignment procedure each of the manually handled heliostat mirrors is excluded from radiation delivery for the duration of the alignment procedure, whereby the amount of collected light is reduced. Moreover, where the heliostat mirrors are composed of facets, the intensity of the individual small light spots produced by each facet of a heliostat mirror that has to be aligned is only a fraction of the total intensity of the full spot produced by that mirror and can be discerned on the background thereof only with great difficulty, which complicates the adjustment of the individual facets of each mirror and gives rise to inaccuracies.